1. Field of the Invention
The disclosure herein relates to an apparatus and method for cleaning semiconductor wafers or other such items requiring extremely high levels of cleanliness.
2. Description of the Related Art
Megasonic cleaning systems are widely used within the semiconductor industry for cleaning substrates such as semiconductor wafers, photomasks, flat-panel displays, magnetic heads or any other similar item that requires a high level of cleanliness. In a megasonic cleaner, the substrates are immersed in or otherwise exposed to a cleaning solution, typically water containing a dilute amount of cleaning chemicals. A source of megasonic energy is activated to agitate the solution and create a cleaning action at the exposed surfaces of the substrates.
In preparing cleaning solutions for megasonic cleaners, gasified water has to date been used only when performing batch cleaning of substrates, such as in an open bath-type megasonic cleaner. For example, U.S. Pat. No. 5,800,626 teaches the use of gasified cleaning solutions in connection with bath-type megasonic cleaning systems. However, gasified cleaning solutions have not been shown to be effective in use with a probe-type megasonic cleaner.
In contrast, with the recent introduction of probe-type megasonic cleaners it has been widely believed unnecessary to gasify the water supplied to the cleaning device, and has even been believed necessary to de-gas the water. Various reasons underlied these beliefs. First, in a probe-type cleaner the megasonic energy source is much closer to the substrate being cleaned; this was thought to obviate the need for any additional cleaning action contributed by the presence or addition of gases dissolved in the solution. In addition, many in the industry believed that the spraying, under pressure, of solution onto the substrate which typically occurs in a probe-type system would aerate the water somewhat and provide the (then presumably modest, if at all) amount of dissolved gas needed for satisfactory cleaning performance with a probe-type cleaner. In other words, it was presumed that the probe-type system would behave in a similar manner as open-tank batch megasonic baths, in which the solution includes gases dissolved from the ambient air. Finally, there has been a fear that excessive oxygen dissolved in the water may react with the hydrogen terminated silicon surface of a substrate and cause, under the high levels of agitation created by a probe-type cleaning system, an unacceptable amount of roughening. As a result, the accepted guideline for probe-type megasonic cleaning processes has been that the water supply contain less than 5 ppb of oxygen in solution.
In light of these widely held beliefs, the inventors were surprised to discover that the gasification of the water used in a probe-type megasonic cleaning system led to substantially improved cleaning performance.